Coating for thermionic cathodes



Patented Oct. 16, 1934 UNITED STATES PATENT OF 1,977,318 COATING'FOR THERMIONIC' CATHODES Jack McCullough; San Bruno, Califl, assignor to 8 Claims.

This invention relates to oxide-coated electron emitting cathodes such as are used-in vacuum or mercury vapor rectifier tubes, amplifying tubes, and the like.

Among the objects of my invention are: To provide a method of forming a strongly adherent oxide coating .upon filamentary or other types of cathodes; to provide a cathode having a large electron emissivity in proportion to its area; to provide a cathode capable of standing a materialdegree of overloading without damage, even in gas-filled tubes; to provide a cathode which may be bent or otherwise manipulated in manufacture after the coating is formed; and to pro- '1 vide a method of forming oxide-coated cathodes tion forming a part of this specification, but I do not limit myself to the embodiment of my inven- .tion herein described, as various forms may be adopted within the scope of the claims.

The use of the alkaline-earth oxides as electron emitters in thermionic tubes of various kinds is well known'in the art. Of these oxides, those of strontium and barium appear to be best adapted for the purpose, giving thehighest electron emission, although other alkaline-earths may be used.

3 There has, however, always been difficulty in apvery thin coatings in order to secure the required effect, which makes the resultant cathode expensive. Furthermore, repeated expansions and contractions of the cathode due to intermittent use,- tend eventually to crack the bond, causing the coating to crack or flake ofi.

In general terms, the coating of my invention is formed by applyinga mixture of alkalineearthoxide and sugar to a metal base or core. The core is then heated, preferably electrically,

1 in the presence of oxygen, first to carbonize and then to ignite the sugar in the mixture. The cathode is then reheated in vacuo, preferably in the process of pumping the tube in which it is to be used, the temperature applied being greater than that atwhich the cathode normally op- Heintz & Kaufman, Ltd, San Francisco, Calif.,

a corporation of Nevada 1 No Drawing. Application May 31, 1932,

, Serial No. 614,629

erates. During this stage of the operation an exothermic reaction takes place in the coating, as evidenced by the formation of high temperature spots on the cathode surface, which gradually spread over the whole coating, the temperature againsubsiding after a few moments tothat due to the applied heat. The cathode is pref erably then aged by being operated at normal emission, after which it is ready for service.- The aging process does not appear to alter mates rially the emissive properties of the coating, its function being to consolidate the bond between the coating and thecathode. If this step be omitted, and the cathode allowed to stand cold for some time after its formation, as occurs if the 7 tube be placed in storage immediately, the coating may loosen and fall off in part or in whole when the tube is first operated. Q In order that the process of my invention may be more completely understood the formationof a'ribbon type filament, such as isused in rectifier tubes, will now be described in detail. The material used for the core of the filament may be nickel, molybdenum, platinum, or any other metal which is suificiently refractory. Because of its ,cheapness and general availability nickel is preferred, and in the-present description it will be assumed that this is the material of the core. Oil or other contaminating materials should be removed from the surface before the process is carried out, but a coating of nickel oxide such as is normally present in greater or less degrees appears to have no harmful effect, andthere is some evidence that it may be of definite advantage. I

A paste is prepared from a mixture of the oxide to be used and a sugar syrup. One mixture which has proved satisfactory contains approximately two parts of strontium carbonate by weight to three parts of barium carbonate. This corresponds approximately to one part of strontium carbonate and two parts of barium carbonate by volume, but the mixture was arrived at empirically as that giving slightly better results, although either salt may be used alone, and the difference 100 in degree of activity does not appear to be great as compared with thosecaused by other factors. The syrup used should be comprised as far as possible of pure sugar and water. Manydif ferent sugars have been used in the laboratory andhave been found .to give satisfactory results. The preferred materials are the invert sugars, perhaps the most satisfactory results having been obtained with a syrup comprising dextrose dissolvedin water and boiled down to the consistency of a fairly thick table syrup, but a cane sugar syrup has also been successfully employed, as have the syrups marketed commercially as corn syrup, e. g., that marketed under the tradename Karo. In this connection it should be noted that the more expensive grades of these corn syrups, which contain added sugar and flavoring matter, are definitely to be avoided, since small amounts of impurities such as those represented by the fiavorings used will poison the final coating and suppress the emission of electrons. The ready availability of the unfiavored syrups has led to their use in the experiments leading to this invention, but it has been found that while they are satisfactory if used from a freshly opened can, some reaction appears to take place after they have been exposed to the air which renders the filament inactive. Where the syrup is freshly prepared from pure sugar, however, these results do not obtain. After the carbonates have been mixed with the syrup to form a stiff paste, the mixture may be kept in closed vessels for a considerable period without deterioration. a

The paste is now applied to the surface of the nickel ribbon, and is preferably rubbed in, using considerable pressure. In production, this may be done by machinery. In preparing small lengths of the filament it is sufiicient to apply the paste with a glass rod using the rod to rub it down to as smooth a surface as possible. Since the mixture is very sticky, a thin and adherent coating is obtained in this manner. It is preferable that the coating be made as thin as possible,

any excess being removed by scraping with the round side of the rod in the laboratory procedure.

The filament is next heated by passing current throught it. The temperature is raised sufficiently high to evaporate the water and cause the sugar to carbonize, but not high enough to cause incandescence of itself. The filament coating, originally white, first turns gray andthen practically black. At this point, an exothermic reaction occurs, the coating on the filament starting to glow, usually first in a single spotwhich spreads through the entire coating, and dying out again in a few seconds. This leaves the filament a grayish white.

There may be some lumps in the coating formed during the evaporation of the water from the syrup. A vigorous rubbing of the filament will remove all of these lumps, leaving a thin, homogeneous, and tightly adherent gray coating upon the filament. This coating is much thinner than the oxide coat usually used, as itcomprises but a single layer applied as thinly as possible, but it'should be noted that the attempt to form a thicker coating upon the filament by repeating the foregoing process results in a materially less satisfactory product.

The filament is now put into position in the tube. It may be bent and otherwise manipulated without particular care, Without destroying its efficaoy in any degree, the coating being so ada herent as to be scraped off only with considerable course of this process the filament is heated, the

current being increased gradually from zero to approximately 20 per cent more than the normal operating current of the filament. 'At approximately this point another exothermic reaction occurs in the coating, hot spots appearing on the surface of the filament which spread out until the uniform. Scratches or other imperfections in the core surface are visible through it, and there is no sign of cracks or scaling. If, however, contamination has been present from any source, the filament may be "poisoned and refuse to emit at normal temperature. If this be the case, approximately 40 per cent excess current, passed through the filament for a period of approximately three minutes, will usually activate the fila- .mentmzausing it to give full emission. Because of the difiiculty of maintaining absolute control overall of the steps of the process, I prefer 'to give the filament this flashing treatment as a matter of routine, since it does no harm to originally active filaments, and since almost all poisoned filaments respond to it. Those which do not may be treated by flashing at still higher temperatures, currents as high as 60 per cent over normal having been used upon occasion, '75 per cent overload being that corresponding to the melting point of the nickel.

The tube is now ready for use, and is sealed off of the pump and based in the usual manner. If the process be discontinued at this point, and the tube immediately placed in storage, the oxide coating on the filament may crack andloosen, flaking off and falling to thebottom of the tube. This may be prevented by aging the tube, operating the filament at normal temperature and with normal plate voltage applied for a period of from one to three hours. This appears to consolidate the bond between the coating and the filament, resulting in :a cathode of high emissivity and ruggedness.

' The emissivity of filaments formed in this manner is extremely high. Tubes have been built and operated for long periods at currents 50 per cent higher than those obtainable inotherwise similar tubes having oxide coated filaments prepared in the usual manner. The filaments are even capable of withstanding. a considerable degree of overload in rectifier tubes of the mercury vapor type, withstanding appreciable positive ion bombardment without deterioration, although exc'essive positive ion bombardment will, of course, result in the physical destruction of the filament as is the case with any type.

An interesting feature of the use of cathodes of thistype is that they appear to be free from the effect of cathode sputter to a singular degree. When these tubes are operated side-by-side with tubes having the usual type of cathodes, the bulbs remain clear even when the cathode of the other tubes have sputtered to such a degree that the bulbs have darkened so as to obscure the tube H elements.

I claim:

1. The method offorming an electron emissive surface on a metallic cathode which comprises the steps of coating said cathode with a mixture of alkaline-earth oxide and sugar, and heating said cathode in the presence of oxygen to oxidize the sugar in said coating.

2. The method of forming an electron emissive surface on a metallic cathode which comprises the steps of coating said cathode with a mixture of alkaline-earth oxide and sugar, heating said cathode in the presence of oxygen to ignition, and removing the excess coating from the surface of said cathode.

3. The method of forming an electron emissive surface on a metallic cathode which comprises the steps of coating said cathode with a mixture of alkaline-earth oxide and sugar, and heating said cathode in the presence of oxygen until an exothermic reaction occurs in said mixture.

4. The method of forming an electron emissive surface on a metallic cathode which comprises the steps of coating said cathode with a mixture of alkaline-earth oxide and sugar, heating said cathode in the presence of oxygen until an exothermic reaction occurs in said mixture, removing non-adherent portions of the mixture from the surface of said cathode, and reheating said cathode in vacuo until a second exothermic reaction occurs on said surface.

5. The method of forming an electron emissive surface on a metallic cathode which comprises the steps of coating said cathode with a mixture of alkaline-earth oxide and sugar, heating said cathode in the presence of oxygen until the coating first darkens and then glows with an exothermic reaction, reheating said cathode in vacuo until additional evolution of heat evidences a second exothermic reaction, and aging said cathode by withdrawing space current therefrom to cause permanent adherence of said coating.

6. The method of forming an electron emissive surface on a metallic cathode which comprises the steps of coating said cathode with a mixture of alkaline-earth oxide and sugar, heating said cathode in the presence of oxygen until the coating first darkens and then glows with an exothermic reaction, reheating said cathode in vacuo until additional evolution of heat evidences a second exothermic reaction, and activating said cathode by heating in vacuo to a temperature materially higher than normal emission temperature.

7. The method of forming an electron emissive surface on a metallic cathode which comprises the steps of coating said cathode with a mixture of alkaline-earth oxide and sugar, heating said cathode in the presence of oxygen until the coating first darkens and then glows with an exothermic reaction, reheating said cathode in vacuo until additional evolution of heat evidences a second exothermic reaction, activating said cathode by heating in vacuo to a temperature materially higher than normal emission temperature, and aging said cathode by operation as an emitter to cause permanent adherence of said coating.

8. The method of forming an electron emissive surface on a metallic cathode which comprises coating said surface with a mixture of alkalineearth oxide and sugar syrup, electrically heating said surface in the presence of oxygen to a temperature sufficient to carbonize said sugar, maintaining said surface at said temperature until oxidation of said carbon causes incandescence thereof, rubbing said surface to remove non-adherent particles of said coating, and gradually reheating said surface in vacuo until a spontaneous rise in temperature occurs and subsides.

JACK MCCULLOUGH. 

